change parser
	- reparsing instead of removing blank nodes
extends tests
	- extend RdfConverterTest
	- new RdfSerializerTest
	- helper class for test functions of wdtk-rdf
implement tests for ValueRdfConverter, rename PropertyTypesImpl
Renamed member for consistency
Merge remote-tracking branch 'origin/rdf-serializer' into rdf-serializer
Several performance improvements for RDF serialization

* Use GZIP instead of BZ2, which is much slower (we can re-code data in
BZ2 later on the console if really needed)
* Use buffers before and after compression
* Perform actual compression in separate threads
Merge branch 'rdf-serializer' of https://github.com/Wikidata/Wikidata-Toolkit into create-parser-of-constraints